The present invention relates generally to multi-emitter laser modules with an integrated cooling system.
Single semiconductor lasers or emitters are compact and are widely used in a wide range of applications. In some applications an optical output power is required that is significantly greater than the output of a single emitter. One solution is to create a module with a plurality of single emitters and to combine the optical output of the individual single emitters into a combined optical output. The term single emitter is used herein. An example of a single emitter is for instance the L4 diode laser module by JDS Uniphase Corporation. However, a source of laser radiation can also be a stack or an array or bar of semiconductor lasers or a plurality of laser diodes. An example of a laser bar is the TruDiode 301 by Trumpf. Both the single emitter, which is a single laser diode and a stack or bar of laser diodes or a plurality of laser diodes will be considered herein to be a single laser source, unless specifically identified differently.
Multiple single laser sources can typically be assembled in a module such that the individual beams of the single sources are optically stacked in one axis to generate a single beam of laser radiation with a much higher optical power level than from the single laser source. There are different known ways to position individual laser sources in a module and combine the individual outputs into a more powerful combined optical output.
Edge emitting laser diodes are known in the art. For illustrative purposes a diagram of such a laser diode 100 is provided in FIG. 1. The diode contains multiple layers, including p and n layers and a radiating layer 101, generally with a long body and a smaller facing side. The layer 101, which may be called the active layer of a laser diode, includes the radiating part of interest of the device 100. The laser radiating part of active layer 101 is determined by factors such as top and bottom electrodes that provide electric current to activate the laser diode into emitting laser radiation. As an example, laser radiation may be emitted from at least part of the active layer 101 into the direction 112. Generally, the emitting apertures of a laser diode are rectangular shape with the long dimension having a size of typically tens to hundreds of microns, while the short dimension is typically one to two microns in size. Diffraction effects cause the emerging radiation to diverge, with the divergence angle being inversely proportional to the size of the aperture. The short dimension of the aperture is comparable to the typical laser diode wavelength of approximately eight hundred nanometers; diffraction effects result in large beam divergence in this, the “fast axis”, direction typically between 40 to 60 degrees (90% power content.) The size of the divergence angle is known as the numerical aperture (NA), the beam having a lower numerical aperture along the direction of the stripe than perpendicular to the stripe. The long dimension of the stripe is known as the slow axis of the laser diode and typical divergence in this axis is in the range from 6 to 10 degrees. This divergence not only depends on the specific design of the diode laser, but also on the operating current for broad area diode lasers.
Collimating lenses are applied to collimate the radiation in the direction of the fast axis and optional the slow axis. For a laser diode bar comprising multiple emitting apertures in one device next to each other, a single lens or a microlens array may be used for collimating the beam in slow axis direction. In general, the fast axis collimators are placed directly or close to the output facet of the laser and may be cylindrical or toroidal in shape. At a further distance than the fast axis collimators, a slow axis collimator is applied to each laser source. Accordingly, the output beams of the individual laser sources are all collimated before being combined. The collimated individual source beams then have to be combined with an optical combiner into a single output beam, by for instance a lens, an optical multiplexer or by a polarizer.
In order to be able to properly collimate the individual beams and properly align the beams to enable combining, the individual laser sources have to be arranged and aligned with optics to keep individual beams separate and limit crosstalk and interference of individual beams before the combining step. For power scaling it is advantageous to limit the effects of heat dissipation by providing cooling capabilities to the individual laser sources and the combined unit or module. One way of cooling is by conductive cooling, wherein the generated heat is conducted through carriers and housings. In addition, one can increase cooling capabilities by providing a forced cooling by providing a flowing cooling medium through the carriers that support the laser sources. Furthermore, in many cases it is advantageous to have the powered laser sources being served by the same current, which can be achieved by arranging the laser sources in a serial network.
The related art provides several approaches to cooling laser modules. High power assemblies deploy mounting a single diode laser or diode laser bar on a heatsinks which may include channels. The heatsink is made typically from copper, ceramic or any combination thereof. Efficient heat transfer, matching the coefficient of thermal expansion to that one of GaAs diode, low electrical impedance and limited (electro-) corrosion are typical requirements. Diode coolers are disclosed in patents, such as U.S. Pat. No. 5,903,583, U.S. Pat. No. 8,345,720, U.S. Pat. No. 7,724,791, U.S. Pat. No. 8,130,807. One approach is illustrated in U.S. Pat. No. 5,870,823 issued on Feb. 16, 1999. This patent discloses a sintered ceramic substrate that includes at least one cooling channel that is internal to the ceramic body. Another approach to cooling a ceramic substrate that hosts a laser source is described in Diode laser modules based on laser-machined, multi-layer ceramic substrates with integrated water cooling and micro-optics, by Alberto Campos Zatarain, May 2012.
However, none of these references describe an effective approach to cooling a multi-emitter stepped module with platforms, as disclosed herein. For cost effective manufacturing, preferably each platform hosts optics, like a mirror, lens and, of course, a laser source. The platforms are stepped relatively to each other and relative to a base plane of a module to allow for effective optical stacking or beam combining of the individual laser diodes or laser sources. The prior art does not teach or suggest an effective structure or method to cool a laser diode assembly having a stepped, preferably unitary, ceramic base.
Accordingly, improved and novel multi laser emitter stepped modules with an effective integrated cooling architecture embodied in a single body are required.